The size of the brain is a frequent topic of study within the fields of anatomy, biological anthropology, animal science and evolution. Measuring brain size and cranial capacity is relevant both to humans and other animals, and can be done by weight or volume via MRI scans, by skull volume, or by neuroimaging intelligence testing.
The relationship between brain size and intelligence has been a controversial and frequently investigated question. In 2021 scientists from Stony Brook University and the Max Planck Institute of Animal Behavior published findings showing that the brain size to body size ratio of different species has changed over time in response to a variety of conditions and events.[1]
As Kamran Safi, researcher at the Max Planck Institute of Animal Behavior and the study’s senior author writes:
“Sometimes, relatively big brains can be the end result of a gradual decrease in body size to suit a new habitat or way of moving—in other words, nothing to do with intelligence at all.”[2]